One example of a conventional spherical coupling structure for a piston and a connecting rod is a structure wherein a spherical joint is configured by screwing together the side of a piston and the side of a coupling rod by using a fastening member, such as is shown in FIG. 10 hereof. Another example that uses a spherical joint as a coupling structure for a piston and a connecting rod is proposed in Japanese Utility Model Laid-Open Publication No. HEI-3-17369. This spherical coupling structure is described with reference to FIG. 11.
FIG. 10 shows a case in which a cup-shaped supporting unit 202 as a protruding unit is formed on the reverse side of the crown 201 of a piston 200; a downward-facing concavity 203, a concave semispherical surface 204, and a female screw 206 are formed in the cup-shaped supporting unit 202; the top half of the small end 208 of a connecting rod 207 is slidably fitted into the concave semispherical surface 204; concave spherical surfaces 212, 212 formed in a holder 211 are slidably fitted into the lower half of the small end 208; and a male screw 214 formed in a fastening member 213 is screwed into the aforementioned female screw 206, whereby the holder 211 is fixed in place in the cup-shaped supporting unit 202, coupling the piston 200 and the connecting rod 207 together.
FIG. 11 shows a case in which a semispherical concavity 221 is formed in the reverse side of the crown of a piston 220, the substantially spherical small end 223 of a rod 222 is fitted into the concavity 221, and a flange 224 for holding the small end 223 is mounted on the surface surrounding the concavity 221 by using a plurality of bolts 226, whereby the piston 220 and the rod 222 are coupled together.
FIG. 12 shows the relationship between the crank angle and the force acting on a piston 181 in an internal-combustion engine. The vertical axis indicates explosive force EF and kinetic force KF (*1) as forces that act on the piston, and the horizontal axis indicates the crank angle (−360° to 360°).
The explosive force EF shown by the dashed line is calculated based on the internal pressure of the combustion chamber of the internal-combustion engine. The explosive force reaches a maximum immediately after the crank angle 0° (explosive top dead center) at which the internal pressure of the combustion chamber reaches a maximum, and since the explosive force EF is always positive, this force constantly acts on the piston 181 in a direction from the top dead center side to the bottom dead center side (this direction is referred to as downward; likewise hereinbelow).
The kinetic force KF shown by the solid line is calculated from the mass of the piston 181, the engine speed, the stroke length of the piston 181, and the length of the connecting rod 182. The kinetic force is negative near a crank angle of 0° (and ±360°), and therefore a kinetic force KF1 acts on the piston 181 in the direction from the bottom dead center side toward the top dead center side (this direction is referred to as upward; likewise hereinbelow). The kinetic force is positive near a crank angle of ±180° (bottom dead center), and therefore a downward kinetic force KF2 acts on the piston 181.
FIG. 13 shows the relationship between the crank angle in an internal-combustion engine and the force acting on the piston 181. The vertical axis indicates forces that act on the piston 181, which are the kinetic force KF shown in FIG. 12, and the resultant force RF of the explosive force EF shown in FIG. 12 and the kinetic force KF shown in FIG. 12. The horizontal axis indicates the crank angle (−360° to 360°).
The resultant force RF of the explosive force and the kinetic force is positive at crank angles of about −300° to 300°, and the resultant force RF acts downward on the piston 181 at such angles. The resultant force RF is negative at crank angles of about −360° to −300°, and also about 300° to 360°, and the resultant force RF acts upward on the piston 181 at such angles. The resultant force in this range reaches a minimum at ±360°.
In addition to the direction of the resultant force RF of the explosive force and the kinetic force described above, the movement direction of the piston 181 and the direction of the kinetic force are also shown as crank angle ranges (1) through (8). The solid arrows indicate the resultant force RF, the dashed arrows indicate the movement direction of the piston 181, and the bold line arrows indicate the direction of kinetic force KF.
For example, in the crank angle range (1), the movement direction of the piston 181 is downward, the direction of the resultant force RF is upward, and the direction of the kinetic force KF is upward.
In FIGS. 10 and 13, when the piston 200 moves within the cylinder, kinetic force acts on the piston 200, the holder 211, and the fastening member 213.
(A) For example, in the crank angle range (1) and the crank angle range (8) of the piston 200, the respective upward resultant forces RF1 and RF2 act on the piston 200, or, specifically, the upward resultant force RF1 or RF2 acts on the screw joint between the male screw 214 and the female screw 206, creating tensile stress. At this time, a large portion of the resultant force RF1 or the resultant force RF2 is kinetic force KF, and a large portion of the kinetic force KF depends on the mass of the piston 200 above the screw joint between the male screw 214 and the female screw 206. Therefore, a large amount of tensile stress is created in the screw joint because of such a large mass.
(B) Also, for example, in the crank angle ranges (2), (3), (6), and (7) of the piston 200, the respective downward kinetic forces KF3, KF4, KF5, and KF6 act on the piston 200, or, specifically, on the screw joint between the male screw 214 and the female screw 206. These kinetic forces KF3, KF4, KF5, and KF6 depend on the mass of the holder 211 and the fastening member 213, and since the mass of the holder 211 and the fastening member 213 is small, the tensile stress created in the screw joint is also small.
As described above, since tensile stress in the examples (A) and (B) repeatedly acts on the screw joint between the male screw 214 and the female screw 206, there are still the problems of increased average stress in the screw joint and decreased durability of the screw joint.
In FIG. 11, stress is repeatedly created in the screw joint between the bolt 226 and the female screw 227 either (C) by the kinetic force resulting from the mass of the piston 220, or (D) by the kinetic force resulting from part of the mass of the flange 224 and the bolt 226. Since the mass of the piston 220 is greater than the mass of the flange 224, the stress created in (C) is greater than the stress created in (D), the average stress is greater, and the durability of the screw joint between the bolt 226 and the female screw 227 is lowered.
Furthermore, in FIG. 10, for example, depending on the machining precision of the concave semispherical surface 204 in the piston 200 and the female screw 206, misalignment may occur between a center line 217 that passes through the center of the concave semispherical surface 204 and that is perpendicular to the crown 216, and the axis line 218 of the female screw 206, and it may prove difficult to fasten the fastening member 213. Improving the machining precision causes costs to increase.
Also, even if there is virtually no misalignment between the center line 217 and the axis line 218, for example, cases may occur in which the holder 211 is mounted in the downward concavity 203 near the end as a result of fastening the fastening member 213; part of the spherical surfaces 212, 212 of the holder 211 comes into close local contact with the lower half of the small end 208 of the connecting rod 207; the gaps between the small end 208 and the concave semispherical surface 204 and spherical surfaces 212, 212 become nonuniform; and the oil film is not formed uniformly.
In FIG. 11, for example, there may be cases wherein the bolts 226, 226 are fastened and the piston 220 is mounted in the flange 224; the flange 224 moves a great distance from a specific position and part of the flange 224 comes into close local contact with the small end 223 of the connecting rod 222; and it is difficult to form an oil film between the small end 223 and the flange 224.
Also, when the piston 220 and the connecting rod 222 are connected by this type of spherical coupling structure, the temperature of the connecting part between the piston 220 and the connecting rod 222 rises and it becomes even more difficult to form an oil film on the connecting part when the connecting part is positioned in the middle of the piston 220 and the concavity 221 is positioned nearer to the combustion chamber.